Power converter systems often employ individual power converters coupled in parallel to provide flexibility, modularity and redundancy. As overall load current requirements change, individual power converters can be added, removed or replaced without replacing the entire power system. Similarly, if an individual power converter fails, it can be replaced as a module.
One of the major concerns regarding proper operation of paralleled power converters is the ability to balance load currents between the power converters. For example, if a power converter system contains two power converters operating in parallel, each power converter is typically tasked to deliver one half of the total load current. If one of the power converters fails to deliver its allotted portion of the total load current, the other power converter is called upon to compensate for the shortfall. There has been extensive research on how to control load sharing among converters, and many control schemes have been developed. These methods are widely used in the power electronic industry, and prove to suffice in many applications.
In a distributed power system, sometimes there is a need to parallel several converter sub-systems, where each consists of several DC-DC converters. This further complicates the problem since each sub-system has its own voltage set point, which is typically used to control the value of its own output voltage. Slight differences among these voltage set points, often due to circuit component tolerances in the sub-systems, would then result in large output voltage imbalances among the DC-DC converters. Therefore, it is usually necessary to use one of the control signals to set the output voltage of the DC-DC converters. This control signal may be obtained from a control bus which is formed by using the voltage set point signal from each converter subsystem to form a logical "OR" circuit.
A conventional way of forming a logical OR circuit for voltage signals uses diodes for implementation. The problem associated with using diodes to form a logical OR circuit is that voltage adjustment is unidirectional, allowing the converter voltage to be only margined up or only margined down. In many practical applications, however, the control circuit has to have the capability to both margin up and margin down, thereby adjusting the output voltage in either direction.
Another important aspect of the output voltage control system is its robustness against failure. In a typical distributed power system, a single control signal failure may bring down the whole system of paralleled DC-DC converters. This occurs because conventional methods, used to select one of the control systems, are sensitive to the other control signals if they should become faulted.
Accordingly, what is needed in the art is a way to conveniently orchestrate the output voltages of paralleled converters in a fault tolerant manner while maintaining the capability of adjusting the output voltage in either direction.